Fine particle magnetic material

ABSTRACT

Fine particle magnetic material prepared by electrodeposition of iron or iron-cobalt alloys into a mercury cathode is coated with a protective coating of antimony and then placed in a lead matrix containing a small but critical percentage of an additive material which may be calcium, tin or a tin-base alloy. The additive material acts to enhance the dimensional stability and retard the growth of whiskers on the resulting magnet.

United States Patent 2,974,104 3/1961 Paine et a1. 252/62.53X

2,999,777 9/1961 Vamartino et a1. 252/62.55X

2,999,778 9/1961 Mendelsohn 252/62.55X

3,073,728 l/l963 Falk 242/62.55X OTHER REFERENCES Wright Powder Metallurgy No. 4, Pages 79- 89, 1959 Primary ExaminerTobias E Levow Assislan! Examiner.l. Cooper AnorneysHarold J Holt, Frank L. Neuhauser, Oscar B.

Waddell and Melvin M. Goldenberg ABSTRACT: Fine particle magnetic material prepared by electrodeposition of iron or iron-cobalt alloys into a mercury cathode is coated with a protective coating of antimony and then placed in a lead matrix containing a small but critical percentage of an additive material which may be calcium, tin or a tin-base alloy. The additive material acts to enhance the dimensional stability and retard the growth of whiskers on the resulting magnet.

. 1 .FINE PARTICLE MAGNETIC MATERIAL BACKGROUND OF THE lNVENTlON This invention relates to fine particle magnetic material having a lead matrix and to a process for preparing such magnetic material.

' Lead has proven to be a uniquely suitable matrix for fine particle magnetic material in which each of the particles has a transverse dimension of a single magnetic domain. Lead, or a lead alloy with up to percent antimony, protects the particles against chemical attack, provides a suitable spacer for the particles and does not have a deleterious effect on magnetic properties. However, lead does have certain drawbacks. Fine particle magnets made with lead matrices have a certain degree of dimensional instability, and in addition are subject, when exposed for prolonged periods to elevated temperatures (above about l30 C.), to a phenomenon which has been signee as the present invention, identifies one method of overcoming the problem of whisker growth. This patent indicates that theaddition to the lead matrix material of at least 0.09

part by weight of cadmium per part of lead overcomes this tendency for the magnetic material to form whiskers. While the addition of cadmium in the percentages indicated in this 1 patent does tend to overcome the whisker problem, there is a practical difficulty encountered with cadmium which makes it difficult to use with known processes for the manufacture of the fine particle magnetic material. Cadmium has a relatively "high vapor pressure, and therefore tends to boil off when subjected to elevated temperatures. In the preparation of the fine particle magnetic material, the matrix material, including any additives, is normally subjected to a vacuum distillation step to vremove the mercury, at a temperature at which the vapor I pressure of cadmium is significant. it has therefore been very difficult in practice to effectively introduce cadmium into the matrices of the fine particle magnetic material. The aforemenftioned Fall; patent also indicates that 0. 10 part by weight of tin per part by weight of lead inhibits whisker growth to some extent but the patent also indicates that tin severely attacks the 1 magnetic particles.

SUMMARY OF THE lNVENTlON it has. now been discovered that the problem of whisker growth with fine particle magnetic material containing a lead matrix may be substantially overcome by the addition of an extremely small-but critical quantity of either calcium, tin, or a tin-base alloy without any deleterious effect on the particles.

Processwise, an important feature of the invention resides in placing a protective coating on the particles prior to the tin addition. The resulting magnet has been found to have a number of improved properties, including dimensional stability, in addition to the substantial absence of whisker growth problems. Although it is not intended to be limited to any specific theory by which the present results are achieved, it is believed that the presence of one of the named additives increases the wettability of the lead matrix for the fine particles and therefore reduces the internal stresses in the magnet. Such internal stresses are known to underlie the nucleation and growth of whiskers. The dimensional instability in the lead matrix fine particle magnetic material is believed to result from oxidation at the powder particle grain boundary. The additives are believed to migrate or diffuse into these grain boundaries and inhibit further internal oxidation. The result is that the magnet has reduced corrosion, increased-strength and dimensional stability and reduced whisker growth.

BRIEF DESCRIPTION OF THE DRAWING The invention will be more clearly understood from the following description taken in conjunction with the accompanying drawing in which:

FIG. 1 is a graph showing the effect on total magnetic energy ((BH) Max.) of the addition of increasing quantities of tin; and

F IG. 2 is a graph of the change in modulus of rupture of tinmodified fine particle magnets with increased heat-aging time.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The fine particle magnetic materials to which the present invention is directed are prepared by electrolytically depositing iron or iron-cobalt alloys into a liquid metal cathode, such as mercury, from an acidic electrolyte comprising ions of the iron or iron-cobalt metals while maintaining a quiescent interface between the cathode and the electrolyte. The particles thus deposited are elongated and have a transverse dimension of a single magnetic domain. After heat-aging, lead and antimony are added to the particle-mercury mixture as a matrix and as an antimony protective coating for the particles. It is important that this protective coating be present on the particles prior to the tin addition. Tin is then added, the mercury is removed and the particles aligned and compacted into a magnet structure. The process of preparing the particles and of coating with antimony and the utilization of a lead matrix are all more fully described in US. Pat. Nos. 2,974,104; 2,999,777 and 2,999,778 respectively.

The preferred matrix additive is tin because of its ready availability and the relative simplicity of its use in the preparation of the present magnetic materials. Reference hereafter to tin is intended, unless otherwise indicated, to be illustrative of the additive material. An important feature of the present invention is the stage of the process in which the tin is added to the mixture containing the fine particle magnetic material. The tin should be added after the addition of the lead-an timony matrix so that the antimony has reacted with the particles to form a protective antimony coating. This protective coating stabilizes the particles andacts as a barrier to the influx of tin atoms which otherwise would deleteriously affect the magnetic properties of the resulting magnet. The tin may conveniently be added in the form of small pellets, as for example of about /a-inch diameter. Following the addition of the tin, the material is preferably compressed while subjected to a magnetic field. This aligns the elongated fine particles in the direction of the magnetic field to obtain optimum ratio of residual-to-saturation induction and also removes a considerable amount of the mercury. The remaining mercury may then be removed from the mixture by vacuum distillation at an elevated temperature. The protective coating on each particle allows the vacuum distillation to becarried out without spheroidizing the magnetic particles and degrading their magnetic characteristics. The temperature of distillation is generally from 300 C. to 400 C., the pressure less than 1 mm. of mercury, and the time of distillation from 1 to 12 hours, depending on the size of the compact. Following distillation, the more or less porous mass of iron or iron-cobalt fine particle magnetic material, antimony, lead and tin is ground and pressed in a directionalizing magnetic field, typically at a pressure of about 50,000 p.s.i. in the presence of a directionalizing field of about 4,000 gauss or more.

The maximum quantity of the additive used for the reduction of whisker growth and for enhancing the dimensional sta bility of the magnet is critical. This criticality is illustrated graphically in FIG. 1 of the drawing, with the preferred additive material, tin. As there shown, the maximum magnetic energy of about 3.25 X 10 gauss oersteds is reached with the addition of up to about 0.02 parts by weight of tin per part by weight of lead matrix. Above this quantity, the maximum magnetic energy drops rather sharply to the point where the maximum magnetic energy is less than 2.9 X 10 oersteds at quantities greater than 0.05 parts by weight of tin. At the lower end of the range, even trace quantities show improvement in physical stability, i.e., quantities as low as about 0.001 parts. At levels below about 0.005 it is difficult to obtain homogeneous and uniform distribution throughout the magnet matrix, but such uniform distribution is possible if sufficient care is taken.

Although unalloyed tin is the preferred additive material, it has been found that certain tin-base alloys also achieve the improved results of the invention. Specifically, alloys of 14.3 percent by weight copper, balance tin, 6.5 percent by weight bismuth, balance tin, also enhance dimensional stability and reduce whisker growth. The maximum proportion of tin-base alloys is, however, somewhat higher than the maximum proportion for tin itself. The tin alloys may be added in quantities up to 0.040 parts by weight per part of lead matrix. Over 0.040, a drop in the magnetic properties of the resulting magnet occurs, similar to that illustrated with tin. Additional tin alloys that are also included within the scope of the present invention are tin-base alloys containing either indium or telluri- To achieve homogeneity in the magnetic composition, it is necessary that the extremely small quantities of the additive be very thoroughly and uniformly dispersed in the magnetic composition. This may be accomplished by a heat-treating step for a relatively brief period of time, after the tin is added, as for example for minutes at about l50200 C. In spite of the extremely small quantities of additive utilized in the magnetic composition, it has been found that this simple homogenizing step uniformly distributes the tin throughout the magnetic composition, and the important advantages in physical stability are achieved.

The following example illustrates the preparation of a magnetic structure in accordance with the practice of the present invention.

EXAMPLE 11 A mercury slurry of fine iron-cobalt particles, electrolytically deposited into mercury as disclosed in the aboveidentified U.S. Pat. No. 2,974,104, was heat-aged for 10 minutes at 195 C. The slurry, prior to heat treatment, contained 477 pounds of mercury and 23 pounds of iron-cobalt particles. While still hot, 55.4 pounds of lead as a matrix material and 3.5 pounds of antimony as a protective coating material were added to the slurry. The resultant mixture was heat-treated for an additional 10 minutes at 195 C. To this slurry was then added 1.00 pounds of tin and the mixture was heat-treated an additional 10 minutes at 195 C. After cooling, the mixture was pressed at a pressure of 10,000 p.s.i. in a nonmagnetic mold in the presence ofa DC magnetic field of 4,000 gauss to align the elongated iron-cobalt particles in the direction of the field, to form preforms of the particles, and to reduce the mercury content to about 80 percent of its original amount. Essentially, all of the rest of the mercury was then removed by distilling the material at a pressure of about 1 mm. of mercury for 4 hours at 350 C. This reduced the mercury to about 2 percent by weight of the preform. The preforms were then ground in a rotary cutter, classified according to size, a small amount of lubricant was added, and the composite was then pressed at a pressure of about 50,000 p.s.i. into magnets having a packing fraction of 32.

The same process set out in the above example was repeated except that an alloy of 14.3 percent by weight copper, balance tin, was used in place of straight tin. A third batch was prepared, as above, but instead using an alloy containing 6.5 percent by weight bismuth, balance tin. Finally, a fourth batch was prepared using calcium instead of tin in a proportion of 0008 parts by weight per part by weight of lead.

Magnets prepared as set out above were then exposed to prolonged heat-aging experiments to determine whether the magnetic properties of the magnets were affected by the addition of the various additives. After exposure for times varying from 352 hours to as long as 532 hours at 90 percent relative humidity, at temperatures from 104 F. to as high as 180 F it was found that the magnetic behavior of magnets with the respective matrix additives set out above was identical to the magnetic properties of identical fine particle magnets prepared as above except that the additives were omitted.

Another series of tests was carried out to determine whether magnets prepared in accordance with the'example set forth above were subject to whisker growth. Specifically, repeated progressive temperature tests in airwerc carried out from 300C. on magnets prepared as set forth-above and on identical magnets prepared as set forth above but without the addition of the in or other matrix additive. No whiskers were observed on magnets with the modified matrix system at temperatures up to 280 C. On magnets without the additive, whiskers were observed to grow at temperatures above 130 C. The invention thus raises the nucleation temperature of the whiskers to 280 C.

It has additionally been discovered that the modulus-of-rupturc strength of magnets is improved by the addition of the tin or other additive in accordance with the practice of this invention. Moreover, heat-aging of the modified matrix magnet, or annealing at an elevated temperature for periods up to 24 hours, even further improves the modulus of rupture. This is best illustrated in the graph of FIG. 2 of the drawing with a small bar magnet, prepared as set forth in Example 1, of 0.3- inch length, 0.07-inch width and 0.03-inch thickness. As there shown, the modulus of rupture (obtained by three-point bending tests) of untreated fine particle magnets is 6,000 p.s.i. The equivalent modulus of rupture of the identical magnet containing tin is shown to be 9,200 p.s.i., which increases upon heat-aging at 200 C. to over 14,000 p.s.i. after 8 hours. At packing fractions above about 35, lower aging temperatures should be used to avoid magnetic degradation. It should be noted, as illustrated in the graph, thatthe modulus of rupture is improved even without any heat-aging. This strength improvement effect with heat-aging cannot be obtained by the unmodified lead matrix material because of whisker growth occurring during the aging treatment.

lclaim:

l. Fine particle magnetic material consisting essentially of single magnetic domain particles selected from the group consisting of iron and alloys of iron and cobalt, each of said particles containing an antimony protective coating and a lead matrix and from a trace quantity of about 0.001 up to 0.04 part by weight per part by weight of lead of an additive selected from the group consisting of calcium, tin, and tinbase alloys, said additive being homogeneously and uniformly distributed throughout said matrix in a quantity sufiicient to enhance dimensional stability and reduce whisker growth of the magnetic material.

2. The fine particle magnetic material of claim 1 in which the additive is from 0.001 to 0.02 part by weight of tin.

3. The fine particle magnetic material of claim l in which the additive is a tin-base alloy.

3. Fine particle magnetic material consisting essentially of elongated single magnetic domain particles selected from the group consisting of iron and alloys of iron and cobalt, each of said particles containing an antimony coating and a lead matrix and from 0.001 to 0.02 part by weight of tin per part by weight oflead.

5. A process for producing fine particle magnetic material comprising electrolytically depositing fine magnetic particles selected from the group consisting of iron and alloys of iron and cobalt into a liquid metal cathode of mercury, coating said magnetic particles with an antimony protective coating material and placing said coated particles in a lead matrix, then adding to said matrix from a trace amount of about 0.001 up to 0.04 part by weight per part by weight of the lead matrix of an additive selected from the group consisting of calcium, tin, and tin-base alloys, said additive being homogeneously and uniformly distributed throughout said matrix in a quantity sufficient to enhance dimensional stability and reduce whisker growth of the magnetic material, separating the cathode from ticles in a lead matrix, then adding to said matrix from 0.001 to 0.02 part by weight of tin per part by weight of the lead matrix, heat-treating for a brief period of time said mixture of mercury cathode, fine particles and matrix at ISO-200 C. to homogenize and uniformly distribute the additive in the matrix, removing by vacuum distillation the mercury cathode from the coated fine particle-matrix mixture, and forming said particles and matrix into a magnet structure. 

1. Fine particle magnetic material consisting essentially of single magnetic domain particles selected from the group consisting of iron and alloys of iron and cobalt, each of said particles containing an antimony protective coating and a lead matrix and from a trace quantity of about 0.001 up to 0.04 part by weight per part by weight of lead of an additive selected from the group consisting of calcium, tin, and tin-base alloys, said additive being homogeneously and uniformly distributed throughout said matrix in a quantity sufficient to enhance dimensional stability and reduce whisker growth of the magnetic material.
 2. The fine particle magnetic material of claim 1 in which the additive is from 0.001 to 0.02 part by weight of tin.
 3. The fine particle magnetic material of claim 1 in which the additive is a tin-base alloy.
 4. Fine particle magnetic material consisting essentially of elongated single magnetic domain particles selected from the group consisting of iron and alloys of iron and cobalt, each of said particles containing an antimony coating and a lead matrix and from 0.00l to 0.02 part by weight of tin per part by weight of lead.
 5. A process for producing fine particle magnetic material comprising electrolytically depositing fine magnetic particles selected from the group consisting of iron and alloys of iron and cobalt into a liquid metal cathode of mercury, coating said magnetic particles with an antimony protective coating material and placing said coated particles in a lead matrix, then adding to said matrix from a trace amount of about 0.001 up to 0.04 part by weight per part by weight of the lead matrix of an additive selected from the group consisting of calcium, tin, and tin-base alloys, said additive being homogeneously and uniformly distributed throughout said matrix in a quantity sufficient to enhance dimensional stability and reduce whisker growth of the magnetic material, separating the cathode from said coated fine particle-matrix mixture and forming said particles and matrix into a magnet structure.
 6. The process of claim 5 in which the additive is from 0.001 to 0.02 part by weight of tin.
 7. A process for producing fine particle magnetic material comprising electrolytically depositing fine magnetic particles selected from the group consisting of iron and alloys of iron and cobalt into a mercury cathode, coating said magnetic particles with a protective antimony coating and placing said particles in a lead matrix, then adding to said matrix from 0.001 to 0.02 part by weight of tin per part by weight of the lead matrix, heat-treating for a brief period of time said mixture of mercury cathode, fine particles and matrix at 150-200* C. to homogenize and uniformly distribute the additive in the matrix, removing by vacuum distillation the mercury cathode from the coated fine particle-matrix mixture, and forming said particles and matrix into a magnet structure. 